1. Field of Invention
This invention relates to remote pressure sensing, specifically using pressure transducers that rotate permanent magnets mechanically from within sealed vessels, including tires, to sensors responsive to the direction of a magnetic field.
2. Description of Prior Art
U.S. Pat. No. 6,620,006 to Burns discloses a remote pressure reporting system comprising (1) an in-tire magnetic pressure sending apparatus wherein a permanent magnet is rotated mechanically in response to pressure plus (2) a remote magnetic pressure reading apparatus mounted containing sensors responsive to magnetic field direction U.S. Pat. No. 6,647,771 also to Burns discloses another magnetically coupled tire pressure reporting system based on a novel magnetooptic display attached to the outer tire wall.
U.S. Pat. No. 3,807,232 to Wetterhorn describes a magnetically coupled pressure readout based on rotating a magnet in response to pressure using a Bourdon tube and sensing that rotation with a magnetic compass. Bourdon gauges are fragile, complex, and produce relatively week forces. Accordingly, Bourdon tube mechanisms are not well suited for supporting appreciable magnet masses and for overcoming friction. In fact, forces generated by the interaction of the earth""s magnetic field with a supported magnet can exceed those available from Bourdon tube mechanisms. Bourdon mechanisms are too fragile for employment in vehicle tires. Additionally, the Wetterhorn transducer does not include a means for temperature compensation or offsetting.
Angular coupling between rotating elements on shafts via magnetic fields is generally well known. For example, U.S. Pat. No. 5,382,792 to Hurst et al, describes a coupling mechanism wherein permanent magnet pairs are incorporated into coaxial shafts to provide an instantaneous indication of the orientation of a rotating shaft internal to a motor vehicle engine. Such coupling mechanisms employ multiple permanent magnets, oriented pole-face to pole-face. In these types of devices, magnetic coupling between the pole faces of paired permanent magnets aligns the xe2x80x9coutputxe2x80x9d shaft with the xe2x80x9cinputxe2x80x9d shaft. To be effective, such mechanisms require narrow gaps between the pole faces of the respective magnet. These types of devices are hermetically encapsulated for protection from environmental debris and require penetration of the engine wall.
U.S. Pat. No. 3,777,565 to Munier et al. describes a sealed water or fluid meter with continuously rotating permanent magnets driven by impellers on input shafts magnetically coupled to magnets on outputs shafts for inducing synchronized rotation. The rotation per unit time of the output shaft indicates the flow rate. Angular displacements (errors) between the xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d shafts are tolerated and even increase torque coupling from the input magnet to the output magnet.
Numerous devices include mechanisms moving a permanent magnet in response pressure or other force to induce a sensed effect in a material responsive to variation in magnetic field strength. For example, U.S. Pat. No. 4,006,402 to Mincuzzi, U.S. Pat. No. 4,843,886 to Koppers, et al, and U.S. Pat. No. 4,627,292 to Dekrone, each describe a device based on either magnetoresistance and magnetic saturation. U.S. Pat. No. 4,339,955 to Iwasaki describes a mechanism that exploits variation in the incremental permeability of a magnetically soft material. Devices based on the sensing the strength of a magnetic field rather than field direction typically require a narrow spacing between the sensor and magnet and are very sensitive to changes in spacing, small misalignments, and extraneous magnetic fields. Accordingly, such devices generally require careful and extensive calibration before measurements are made.
U.S. Pat. No. 4,866,982 to Gault describes a tire pressure monitoring system where a stationary Hall-effect sensor measures spacing between a fixed magnet and a second magnet moveable in response to a linear pressure actuator. Changes in spacing between the magnets affect features of the combined magnetic field pattern. Variation in the combined pattern is determined from signal waveforms generated as the spaced magnets, rotating with a wheel, sweep by a stationary sensor. This device requires close coupling between magnet and sensor and penetration into the pressurized interior of the tire and rim. There are no temperature compensation or offsetting means.
U.S. Pat. No. 5,814,725 to Furuichi et al. describes a mechanism that penetrates a tire rim wherein a piston-driven screw rotates a permanent magnet. The strength of the magnetic field is detected by a stationary Hall-effect sensor that is mounted transversely to the magnet rotation axis. This type of device typically shares the same problems as the other devices that depend on sensing magnetic field strength rather than rotation. Again, there are no temperature compensation or offsetting means.
U.S. Pat. No. 5,047,629 to Geist describes a hermetically sealed mechanism for sensing linear displacements of a ferromagnetic armature (e.g., a single turn in a coil spring) according to the attractive force on freely rotating magnet. Disadvantages inherent in this type of device relate to the small distances required between the armature and the magnet, to the small amount of rotational displacement of the magnet produced, and to inadvertent magnetization of the armature.
Other examples of remote pressure reporting mechanisms involve changes in electromagnetic induction or inductive coupling between active elements. For example, U.S. Pat. No. 5,455,508 to Takahashi utilizes a form of time-varying (alternating current) electrical excitation. Disadvantages of these types of devices relate to the need to provide a source of operating power within the pressure container and to inadvertent production of eddy currents in nearby conductive materials that will distort the desired field. These types of devices do not sense magnetic field direction.
Still other concepts of remote pressure sensing involve a change the state indicator responding a preset pressure level. For example. U.S. Pat. No. 3,946,175 to Sitabkhan describes switching a magnetically susceptible need in response to pressure actuated displacement of a magnet. U.S. Pat. No. 5,542,293 to Tsuda et al. describes a conventional bellows actuated mechanism that uses a fixed and a moveable magnet to switch the orientation of a third magnet. U.S. Pat. No. 5,717,135 to Fioretta et al. describes use of magnetic coupling to switch the state of a transducer from producing to not producing a signal. These types of mechanisms are incapable of producing a continuous output responsive to pressure.
Other examples of remote monitoring of vehicle tire pressure involve wireless or telemetric transmission or data. For example, U.S. Pat. No. 5,960,804 to McClelland describes a radio transmitter that sends data collected and stored in a memory device within a tire to an external receiver. This active device requires a source of electrical energy (a battery) inside the tire. Alternatively, U.S. Pat. No. 6,053,038 to Schramm et al. proposes a scheme where an external oscillator circuit generates electromagnetic signals coupling to and energizing a second oscillator within the tire, which changes state responsive to tire pressure and/or other sensed parameters.
Several mechanisms besides Bourdon tubes have been proposed for converting pressure or force into rotary motion, For example, U.S. Pat. No. 4,307,928 to Petlock describes a helical bellows for imparting rotational displacement when compressed mechanically in order to make an improved electrical contact. A single, high pitch helical lead is employed because the desired rotational translation is small. U.S. Pat. No. 5,103,670 to Wu describes the use of a spiral screw to convert linear displacement from a conventional bellows to actuate a directly viewed rotary dial or pointer. U.S. Pat. No. 6,082,170 to Lia at al. describes a blood pressure apparatus that uses a diaphragm bellows and a compressible helical ribbon spring to rotate a dial pointer. None of these types of device employs magnetic coupling for remote sensing. Once again, there are no temperature compensation or offsetting means.
In accordance with the present invention a temperature compensated or offsetting magnetically coupled pressure sender comprises a pressure responsive element coupled to a rotating permanent magnet by means of a temperature sensitive element. The sender is intended to relay a pressure reading from within sealed vessels, including vehicle tires, without requiring penetration the wall of the pressure vessel. In vehicular applications, the recommended pressure is usually stated for xe2x80x9ccoldxe2x80x9d tires. Operating a vehicle heats up its tires and increases tire pressure, which may produce an erroneous indication. Pressure senders described in this invention have temperature-compensating or offsetting elements built in to the pressure senders so that the indicated pressure is always referenced to a xe2x80x9ccoldxe2x80x9d tire.
The primary advantage of the invented sender relates to the ease of providing compensation for temperature effects on the pressure within a sealed container as reported by a magnetically coupled pressure transducer.
Other advantages of the invented temperature-compensating sender are in its simplicity and robustness.
Still other advantages of the invented temperature-compensation sender are in its small size and weight.
A particular utility of the invented temperature-compensating sender is that vehicular safety is enhanced substantially by accurate tire pressure reporting that compensates for tire temperatures differing from the xe2x80x9ccoldxe2x80x9d reference value.
Still further advantages will become apparent from a consideration of the ensuing description and accompanying drawings.